This invention pertains to the field of semiconductor photodetectors, and in particular, to Si-based photodetectors enhanced by implementation of silicide waveguides for high speed and high quantum efficiency light detection.
High-speed detectors and detector arrays are required for telecommunication systems, for high-capacity local area networks, and for instrumentation. For chip-to-chip or processor-to-processor data exchange, optical interconnects will eventually replace today""s electrical methods. Many such approaches are being explored. As the number of detectors required for each application increases, low material cost, mature fabrication technology and compact design present many advantages. Compared to detectors requiring surface normal coupling, waveguide detectors are simpler to assemble.
Traditionally photodetectors with high speed and high quantum efficiency have been limited to materials with direct band-gap, such as III-V compounds. It is desirable to fabricate such devices with silicon-based materials, to benefit from the low cost and mature silicon technology. Because the light absorption in the near IR range for silicon (Si) is low it is necessary to uncouple the optical path length (quantum efficiency factor) and the distance between electrodes (speed limit factor).
Metal-semiconductor-metal (MSM) detectors are often proposed as an alternative for high-speed operation, as the speed is primarily limited by the transit time between electrodes. The distance between photodetector electrodes can be scaled down through fine line lithography. MSM detectors are described in the following publications: xe2x80x9c140 GHz metal-semiconductor-metal photodetectors on silicon-on-insulator substrate with a scaled active layerxe2x80x9d, M. Y. Liu, E. Chen, and S. Y. Chou, Appl. Phys. Lett. 65(7), p. 887, 1994; xe2x80x9cHigh-speed metal-semiconductor-metal photodetectors fabricated on SOI-substratesxe2x80x9d, K. Honkanen, N. Hakkarainen, K. Mxc3xa4xc3xa4ttxc3xa4, A. Kilpela and P. Kuivalainen, Physica scripta T79, p. 127, 1999; xe2x80x9cComparison of the picosecond characteristics of silicon and silicon-on-sapphire metal-semiconductor-metal photodiodesxe2x80x9d, C. C. Wang, S. Alexandrou, D. Jacobs-Perkins, and T. Y. Hsinag, Appl. Phys. Lett. 64(26), p.3578, 1994. The contents of the aforementioned publications are incorporated here in by reference.
To increase the optical path length, and therefore the quantum efficiency, several approaches have been explored. Waveguide geometries, often formed on silicon-on-insulator (SOI) substrates as silicon ridges, have been used, as described in xe2x80x9cSOI waveguide GeSi avalanche pin photodetector at 1.3 xcexcm wavelengthxe2x80x9d, T. Yoshimoto, S. G. Thomas, K. L. Wang and B. Jalali, IEICE Trans. Electron., E81-C(10), p.1667, 1998, and xe2x80x9cNear-infrared waveguide photodetectors based on polycrystalline Ge on silicon-on-insulator substratesxe2x80x9d G. Masini, L Colace, G. Assanto, Optical Materials 17 (2001) 243-246, incorporated here in by reference. The collection of carriers is accomplished either by forming MSM structures on the surface of the ridge waveguides, or by grown-in pin structures.
Another approach is to use vertical cavity structures, which consist of a thin absorbing layer sandwiched between two mirrors. In the past, vertical cavities have been used successfully to build high speed, high quantum efficiency photodetectors. The mirrors were made of dielectric/silicon or Si/SiGe multi-layers through deposition. This method is described in the following publications: xe2x80x9cSelective epitaxial growth Si resonant cavity photodetector xe2x80x9d, G. W. Neudeck, J. Denton, J. Qi, J. D. Schaub, R. Li and J. C. Campbell, IEEE Photo. Technol. Lett. 10(1), p. 129 (1998) and xe2x80x9cSi/SiO2 resonant cavity photodetectorxe2x80x9d, D. C. Diaz, C. L. Schow, Jieming Qi, J. C. Campbell, J. C. Bean and L. J. Peticolas, Appl. Phys. Lett. 69(19), p. 2798 (1996).
Buried silicide layers, embedded by wafer bonding or implant, have also been used as mirror material for vertical cavities, as described in: xe2x80x9cA vertical cavity longwave infrared SiGe/Si photodetector using a buried silicide mirror,xe2x80x9d R. T. Carline, D. A. O. Hope, V. Nayar, D. J. Robins and M. B. Stanaway, Technique Digest of IEDM""97, p. 36.1.1 (1997) and in xe2x80x9cFabrication of ultra-fast Si-based MSM photodetector,xe2x80x9d M. Lxc3x6ken, L., S. Mantel and Ch. Buchal, Electron. Lett. 34 (10), p. 1027, 1998. The contents of the aforementioned publications are incorporated here in by reference.
However, the devices described in the prior art present several limitations described below.
In the case of MSM detectors, only the photocarriers generated near the electrodes (i.e. near the surface) can be collected through drift in the electric field. For carriers generated outside the electric field, a portion of the carriers can be collected through diffusion rather that drift, resulting in a reduction in speed. Forming MSM structures on SOI substrates with thin silicon layers may eliminate this problem. However, the responsivity becomes very low.
Waveguide structures with vertical pin junctions require a specific layer sequence, therefore are costly to make and can limit the possibilities for component integration.
For the vertical cavity structures, light is coupled in from the surface normal direction, and these detectors operate only at a set of discrete resonant wavelengths. The resonant wavelength is determined by the distance between the two mirror surfaces. To obtain high quantum efficiency at the designed wavelength, stringent control in layer thickness is required, which often is a challenge in itself.
In view of the potential advantages of a Si-based photodetector, it would be very advantageous to provide a relatively simple Si-based photodetector with high quantum efficiency and fast response. This application discloses such a photodetector.
It has been long desired to design photodetectors in which photogenerated carriers travel perpendicular to the direction of the light propagation, so that speed and quantum efficiency may be optimized independently. Utilizing the unique properties of silicides, the proposed method provides a simple and elegant way to implement such designs.
The first aspect of the invention is a photodetector comprising two separated silicide regions on a substrate and a waveguide of a silicon-based material formed between side-walls of the two silicide regions.
The second aspect of the invention is a photodetector comprising two separated silicide regions on a substrate and a waveguide of a silicon-based material formed between side-walls of the two silicide regions, wherein the silicon-based material is one of a group of materials comprising: silicon, amorphous silicon, silicon germanium, and amorphous silicon germanium.
The third aspect of the invention is a photodetector comprising two separated silicide regions on a substrate and a waveguide of a silicon-based material formed between side-walls of the two silicide regions, wherein the two silicide regions are produced using a metal from a group of metals comprising: nickel, platinum, tungsten, and cobalt.
The fourth aspect of the invention relates to a surface silicidation method of producing a photodetector having a waveguide of a silicon-based material, comprising steps of:
a/ depositing a metal layer on a silicon-based material layer of a substrate;
b/ etching to selectively remove unwanted regions of the metal layer; and
c/ heating the metal layer to induce a metal-silicon reaction to produce at least two separated silicide regions, the silicide regions forming the waveguide of silicon-based material therebetween.
The fifth aspect of the invention relates to a ridge side-walls silicidation method of producing a photodetector having a waveguide of a silicon-based material, comprising steps of:
a/forming a ridge in the silicon-based material layer of a substrate and applying a mask on top of the ridge;
b/depositing a metal layer on the silicon-based material layer of the substrate;
c/ heating the metal layer to induce a metal-silicon reaction to produce at least two separated silicide regions, the silicide regions forming the waveguide therebetween; and
d/etching to selectively remove unwanted metal from the mask without affecting the silicide regions.